In many industries, such as the furniture industry, it is essential that the moisture content of the lumber or wood be accurately controlled. The drying or seasoning of lumber is usually accomplished by placing the lumber in a kiln in which hot air is circulated around the lumber and the wet and dry bulb temperatures of the circulating air are closely controlled to achieve the desired amount of moisture reduction in the lumber. Despite the fact that kiln drying of lumber requires a considerable length of time, this system of drying or seasoning the lumber is still in widespread use.
For a number of years there have been several proposals for drying or seasoning of lumber by dielectric heat in order to reduce the time required for drying the lumber. In certain instances, such as disclosed in U.S. Pat. Nos. 2,543,618 and 3,721,013, it has been proposed that the initial drying of the lumber be carried out in a kiln and that the lumber be dielectrically heated during the final stages of the drying or seasoning operation. While the cost of drying lumber is reduced by the combination of kiln and dielectric drying disclosed in these patents, the cost of the equipment is high and the most economical drying of the lumber is not achieved. A massive kiln housing must be provided to dry lumber in accordance with each of these patents. The lumber is stacked on a kiln truck and rolled into and out of the kiln housing on rails so that the drying apparatus is not easily moved from one location to another.
U.S. Pat. No. 2,387,595 discloses the drying of a single wood bowling pin by encasing the bowling pin in a sheath of rubberized material, applying subatmospheric pressure to the bowling pin and dielectrically heating the bowling pin to reduce the moisture content thereof. The moisture is withdrawn from the bowling pin in a direction which is parallel to the direction of the grain of the wood or longitudinally of the length of the bowling pin. The drying of a single bowling pin substantially increases the cost and limits the application of the apparatus and method disclosed in this patent to a very limited field. The sheath of rubberized material in direct contact with the wood tends to trap moisture on the outer surface and prevents the uniform drying of the wood.
It has also been proposed to decrease the time required in the drying or seasoning of lumber by placing a stack of lumber in a suitable pressure vessel and subjecting the stack of lumber to subatmospheric pressure while dielectrically heating the lumber. For example, an apparatus and method of this type is disclosed in U.S. Pat. No. 3,986,268. However, the cost of providing a pressure vessel suitable for the reception of the stacks of lumber is very expensive and therefore discourages widespread use of this type of method for drying lumber. Also, the weight and size of the pressure vessel prevents easy movement of the pressure vessel from one location to another.